1. Field of the Invention
The present invention relates to an optical encoder for use in measuring displacement or angle.
2. Related Background Art
An optical encoder has a main scale on which a first optical grating is formed, an index scale on which a second optical grating is formed, the index scale being opposed to the main scale, and a light source for illuminating the main scale with light.
The optical encoder is provided with a light receiving element for receiving light that has been transmitted or reflected by the optical grating of the main scale and transmitted through the optical grating of the index scale.
Use of a light receiving element array that also functions as an index scale in this type of optical encoders has already been proposed by Japanese Patent Publication No. 6-056304
The inventor of the present invention has filed patent applications, for example Japanese Patent Application Laid-Open No. 2003-161645, concerning encoders having the above-described structure.
FIG. 12 shows the structure of an reflection type optical encoder, in which a scale is formed by using a micro roof mirror array to enhance utilization efficiency of light.
A structure that uses such a micro mirror array is disclosed in Japanese Patent Application Laid-Open No. 2002-323347.
This structure includes a light emitting element 1, a light receiving element 2 and a scale 3 as a moving member that uses micro roof mirrors.
Light emitted from the light emitting element 1 is reflected by the scale 3 including reflecting portions and non-reflecting portions arranged with small intervals to form bright portions and dark portions distributed on a row of the light receiving element 2.
In connection with this, if the scale 3 is not a structure using the micro roof mirror array but a structure including simple reflecting and non-reflecting portions, a similar distribution of bright and dark portions is formed on the light receiving element 2.
The encoder having this structure is referred to as an incremental type encoder, in which pulses are output in accordance with movement of the scale 3 and it is possible to detect the displacement amount of the scale 3 by counting the pulses.
The incremental type encoder has the problem that the absolute rotational angle position cannot be detected and it is necessary to provide a separate sensor for detecting the absolute position in addition to the encoder.
To overcome this problem, the following structure has been devised.
FIG. 13 shows a scale for detecting the absolute position in such incremental type encoders.
Specifically, radial reflecting portions are arranged on a scale 4 at regular intervals. In this scale 4, a missing portion 6 in which the reflecting portion is missing is partly formed.
When this scale 4 and a detection sensor using a photo diode array are used in combination and an analogue signal is read timed with a digital signal, points of change in the amplitude and central voltage of the analogue signal shown in FIG. 14 can be determined.
At the time when the missing portion 6 of the scale 4 passes by the detection sensor, contrast on the detection sensor decreases, which leads to a decrease in the amplitude of the signal. In other words, changes in the signal level of the central voltage occur when the missing portion 6 of the scale 4 enters and gets out of the area of the detection sensor.
FIG. 15 shows a waveform of the central voltage in an enlarged manner. Based on this waveform, it is possible to detect a changing amount of detected signals accurately.
When the missing portion 6 passes through a predetermined detecting position while a light quantity feedback operation is effected to make the light quantity constant, a decrease in the light quantity occurs.
To compensate the decrease in the light quantity, current supplied to an LED may be increased.
At that time, the current comes to a peak, and it is also possible to detect the original point by detecting the change in the current value.
Alternatively, a point at which the differential value of the signal becomes zero may be determined instead of using the means for detecting the above mentioned peak of the varying signal.
However, in the absolute position detection means of the optical encoder that has been described above as a prior art, although the position of the original point can be determined, there is a problem that the absolute position of the original point can be displaced due to a variation in the positional relationship between the detection head and the scale that may be caused, for example, by a change in the environmental temperature.